Electrostatic spray coating typically involves atomizing a liquid and depositing the atomized drops in an electrostatic field. The average drop diameter and drop size distribution can vary widely depending on the specific spray coating head. Other factors such as the electrical conductivity, surface tension and viscosity of the liquid also play an important part in determining the drop diameter and drop size distribution. Representative electrostatic spray coating heads and devices are shown in, e.g., U.S. Pat. Nos. 2,685,536; 2,695,002; 2,733,171; 2,809,128; 2,893,894; 3,486,483; 4,748,043; 4,749,125; 4,788,016; 4,830,872; 4,846,407; 4,854,506; 4,990,359; 5,049,404; 5,326,598; 5,702,527 and 5,954,907. Devices for electrostatically spraying can-forming lubricants onto a metal strip are shown in, e.g., U.S. Pat. Nos. 2,447,664; 2,710,589; 2,762,331; 2,994,618; 3,726,701; 4,073,966 and 4,170,193. Roll coating applicators are shown in, e.g., U.S. Pat. No. 4,569,864, European Published Patent Application No. 949380 A and German OLS DE 198 14 689 A1.
In general, the liquid sent to the spray coating head breaks up into drops due to instability in the liquid flow, often at least partially influenced by the applied electrostatic field. Typically, the charged drops from electrostatic spray heads are directed by electric fields towards an article, endless web or other substrate that moves past the spray head. In some applications, the desired coating thickness is larger than the average drop diameter, the drops land on top of one other, and they coalesce to form the coating. In other applications, the desired coating thickness is smaller than the average drop diameter, the drops are spaced apart at impact, and the drops must spread to form a continuous voidless coating.